Dental caries are one of the most prevalent chronic infectious diseases in the world. Over half of U.S. children age 5-9 have at least one cavity or filling; by age 17, nearly 80% of our young people have had a cavity. U.S. Department of Health and Human Services. Oral Health in America: A Report of the Surgeon General—Executive Summary. Rockville, Md.: US Department of Health and Human Services, National Institute of Dental and Craniofacial Research, National Institutes of Health, 2000.
Annual expenditures on the treatment of dental caries in the U.S. are estimated to be $40 billion a year according to the Dental, Oral and Craniofacial Data Resource Center. Tooth decay is characterized by the demineralization of enamel and dentin, eventually resulting in the destruction of the teeth. Dietary sugar is often misperceived as the cause of tooth decay; however, the immediate cause of tooth decay is lactic acid produced by microorganisms that metabolize sugar on the surface of the teeth. Studies suggest that of the approximately 700 oral microorganisms, Streptococcus mutans, a bacterium found in virtually all humans, is the principal causative agent in the development of tooth decay. Residing within dental plaque on the surface of teeth, S. mutans derives energy from carbohydrate metabolism as it converts dietary sugar to lactic acid which, in turn, promotes demineralization in enamel and dentin, eventually resulting in a cavity. The rate at which mineral is lost depends on several factors, including the number of S. mutans cells that are present and the frequency and amount of sugar that is consumed.
Therapeutic regimens that take advantage of bacterial interference to replace a pathogenic bacterial strain such as S. mutans with a non-pathogenic, effector strain are known as replacement therapies. Successful replacement therapy requires an effector strain that: 1) is non-pathogenic, 2) alters the microenvironment to prevent colonization or outgrowth of a pathogenic organism, and 3) persistently colonizes the host at risk to prevent reinfection by the target pathogenic organism, and aggressively displaces the pathogenic organism from the tissues at risk in the case where the pathogen is part of the host's indigenous flora.
Application of the principles of replacement therapy requires the isolation of a non-cariogenic effector strain of S. mutans, e.g., an S. mutans strain deficient in lactic acid synthesis that can outcompete native S. mutans in the oral cavity of the host. There is a need in the art for stable, lactic acid-deficient, non-cariogenic strains of S. mutans that can persistently colonize and aggressively outcompete native S. mutans in the oral cavities of the hosts, and that are suitable for use in a replacement therapy in the prevention and/or treatment of dental caries.
The ability of an effector strain to preemptively colonize the human oral cavity and aggressively displace indigenous wild-type strains was initially thought to be a complex phenomenon dependent on a large number of phenotypic properties. However, it was discovered that a single phenotypic property could provide the necessary selective advantage. A naturally occurring strain of S. mutans was isolated from a human subject that produces a lantibiotic called MU1140, which is capable of killing virtually all other strains of mutans streptococci against which it was tested. See e.g., Hillman et al., Infect. Immun. 44:141 (1984). Mutants were isolated that produced no detectable MU1140 or that produced approximately three-fold elevated amounts. The mutants were used in a rat model to correlate lantibiotic production to colonization potential. It was found that the ability of these strains to preemptively colonize the host and aggressively displace indigenous strains of S. mutans increased significantly as the amount of MU1140 produced increased.
The same relationship between MU1140 production and colonization potential was observed in human subjects, where repeated exposures to the wild-type parent strain were required to achieve persistent colonization (Hillman et al. J. Dent. Res. 66:1092 (1985)), whereas a single exposure to the strain producing three-fold elevated amounts of MU1140 was sufficient (Hillman et al. J. Dent. Res. 66:1092 (1987)). The latter strain required over a year to completely replace indigenous strains of S. mutans in the mouths of the human subjects. During this period, it is presumed that their susceptibility to dental caries persisted until the levels of indigenous S. mutans decreased below a threshold level.
In order to further increase the colonization potential of an effector strain for replacement therapy of dental caries, it is desirable to obtain one or more strains of S. mutans that produce elevated amounts of MU1140 or produce variants of this molecule with increased specific activity. Such strains would reduce the period required for the effector strain to eliminate indigenous, lactic acid-producing strains and thereby achieve full effectiveness. Such strains are also more likely to overcome any inherent resistance to colonization, which, while not currently known, may exist in certain individuals in the population being treated. See, e.g., Hillman, Antonie van Leeuwenhoek 82: 361-366, 2002.